The gel microdrop (GMD) secretion assay involves encapsulating cells in a biotinylated matrix, followed by capture and detection of cell-secreted molecules with fluorescent markers (17-34). This technology differs from other encapsulation methods in that the small size of the microdrop (e.g., <50 μm diameter) creates a defined microenvironment around the cell without impeding diffusion of nutrients, antibodies, or nucleic acid probes into the microdrops, or diffusion of secreted products. Furthermore, microdrops can readily be analyzed using flow cytometry and sub-populations can be detected. The number of occupied cells in each microdrop preparation depends on the number of cells used for encapsulation and is approximated by Poisson statistics for single cell encapsulation (19). To obtain microdrops having a high probability of initially containing 0 or 1 cells, an experimental protocol has been developed in which 1-1.5 million cells are encapsulated in 20 million microdrops, resulting in approximately 5-10% single cell occupation . . . . The emulsion is transiently cooled, causing the drops to gel. Once gelled, the microdrops are physically distinct and robust and can be removed from the oil into an aqueous medium by low speed centrifugal separation. Since the microdrop agarose matrix is a permeable semi-solid support, immunochemical procedures can be performed on encapsulated cells.